Use of molten salt fluxes in the secondary aluminum industry is known to improve direct recovery of aluminum in remelting processes. Aluminum and scrap aluminum, such as used beverage containers (UBCs), are treated using such processes. Remelting of the aluminum in a furnace is carried out under cover of a layer of molten salt to prevent oxidation of the aluminum in the furnace atmosphere and to promote coalescence of the molten aluminum so as to maximize recovery of aluminum. During processing, an oxide film tends to form on the surface of the molten aluminum droplets. The oxide film inhibits coalescence of the molten aluminum, causing smaller particles to be lost in the process thereby reducing the amount of aluminum recovered. The unrecoverable aluminum droplets having the oxide film are sometimes referred to as dross.
Use of a salt flux in the furnace helps to strip away and suspend the oxide film so that coalescence of the droplets increases and dross formation decreases. The salt flux wets the oxide film and initiates disintegration of the film, stripping it from the surface of the molten aluminum droplets. Fragments of the oxide film stripped from the aluminum remain suspended in the flux. The aluminum droplets, which have a density greater than the flux, then form a continuous molten pad beneath the flux layer. The flux also prevents further oxide formation by keeping the metal protected from the atmosphere of the furnace.
A typical salt flux is primarily composed of a mixture of high purity sodium chloride and potassium chloride. The high purity salts used in such processes are solution mined and purified by complex, highly developed methods which can drive up the price of the salts. Thus, although by purification it is possible to avoid the harmful effects of sulfate impurities which are present initially in these alkali metal salts, use of high purity salts can be quite costly.
It is desirable to minimize sulfate impurities in salts because the sulfate impurities act as an oxidant which contributes to the formation of the oxide film on the surface of the molten aluminum. The film is formed according to the following reactions: EQU 32/3 Al+4CaSO.sub.4 .fwdarw.CaS+13/3Al.sub.2 O.sub.3 +Al.sub.2 S.sub.3 +3Ca O EQU 8Mg+2CaSO.sub.4 .fwdarw.6MgO+MgS+CaO.MgO+CaS
The formation of film by these reactions results in metal weight loss. Additionally, the formation of oxides and sulfides increases dross formation. Thus, although a standard purity salt may be less costly then a high purity salt, aluminum recovery can be greatly reduced when a standard purity salt is used, because of the harmful effects of the sulfate impurities.
An object of this invention is to provide a low cost composition for improved aluminum recovery in a recycle process.
Another object of the invention is to provide a salt flux composition which includes a standard purity salt for use in any standard process of recycling aluminum, especially UBCs, without the harmful side effects of sulfate impurities.
Yet another object of the invention is to use a salt flux composition, which includes a standard purity salt, a carbon source, an alkaline agent and a fluoride source, in the recycle of scrap aluminum, such as UBCs, to increase the coalescence of the remelted molten aluminum, thereby improving recovery of the metal.
Further objects and advantages of the invention will be found by reference to the following specification.